Digital Video Broadcasting (DVB) is a suite of open standards providing technical guidelines for the compression and transmission of digital audio and video data for digital television. Current DVB—Terrestrial (DVB-T) standards, designed specifically for digital terrestrial television systems, support various source coding techniques for video compression including Moving Picture Experts Group 2 (MPEG-2) compression. These standards include the transmission of signals using Orthogonal Frequency Division Multiplexing (OFDM) modulation, a multi-carrier modulation scheme where symbols are modulated onto a large quantity of orthogonal sub-carriers. An OFDM symbol is made up of multiple sub-carriers, where each sub-carrier occupies a separate narrowband sub-channel of a wideband channel, and each sub-carrier is modulated with a single-carrier modulation scheme, such as quadrature amplitude modulation (QAM). By employing many slowly-modulated narrowband sub-channels, OFDM modulation is more robust than single-carrier modulation schemes to certain channel conditions including narrowband co-channel interference and multipath fading. As a result, OFDM systems can generally employ less complex equalization techniques at the receiver.
FIG. 1 illustrates a conventional receiver front-end in a DVB-T system, which typically includes an antenna or input 101 for receiving analog signals containing the compressed video data; a tuner 102 for amplifying, filtering and down-converting received analog signals to an intermediate frequency (IF); an analog-to-digital converter (ADC) 104 for sampling and digitizing received analog signals; and a down converter 106 for converting digital signals to baseband. Down converter 106 is typically followed by a DVB-T demodulation stage and an error correction stage (not shown).
While OFDM modulation is generally more resistant to channel interference than single-carrier modulation schemes, DVB-T systems and other communication systems employing OFDM are still vulnerable to distortion effects in received signals due to short term random impulse noise (IN) interference at the receiver. In general, IN occurs for short periods of time, and is unpredictable. For example, IN may result from interfering radio signals from other communication systems, or electrical signals generated by nearby electrical devices. The mitigation of IN has been recognized as a major performance issue in the design of DVB receivers, and is used as a key factor in evaluating the performance and quality of DVB receiver devices.
IN detection and suppression (INDS) techniques have been proposed in order to reduce the effects of IN. FIG. 2 illustrates an existing DVB-T receiver front-end 200 employing INDS by way of IN detector 208 and IN suppressor 210. In general, IN detector 208 considers each digital received signal sample provided by ADC 104 individually to determine if IN is present in the sample. Existing techniques for detecting IN in a signal sample include, for example, evaluating the probability density function (PDF) of the sample noise and evaluating the time correlation of consecutive impulse events. If IN is detected in a sample, IN detector 208 notifies IN suppressor 210, which selectively adjusts the amplitude of the corresponding sample using, for example, amplitude clipping that limits the maximum sample value, or nulling that sets the sample value to zero. The adjusted (suppressed) sample is then provided to down converter 106 for further processing.
One major disadvantage of existing INDS systems, including the one illustrated in FIG. 2, is a degradation in the quality of the received signal when IN is not present. Existing INDS systems are designed to suppress IN when it is detected. However, false detection of IN can occur causing erroneous suppression and distortion of the received signal. Additionally, existing INDS implementations tend to be fixed and do not adapt detection and suppression parameters based on the frequency of IN in order to reduce the false detection rate. Accordingly, there is a need for improved INDS design that can effectively reduce the effects of IN when present in the received signal, without sacrificing performance of the receiver in the absence of IN.